I believe it's deeply serious, and the scientifically correct stance. Especially the observation:

"Claude exhibits markers in its behaviors, self-reports, and internal representations that we would consider welfare-relevant if observed in biological organisms."

is undeniably true in my opinion. If you use the established methods by which we judge animals to be conscious, then it's hard to argue that LLMs are not. That might be an issue with the methods, but it seems clear that you can't rule it out as such.

Keep in mind that animals were also not necessarily considered conscious.

You seem to intuitively disagree? What's your reasoning?

I tend to think of it as reappropriating words in a different context. Since we're talking about language models, they're analogues but not as we would assign the same meaning to other humans.

It's marketing that some of them have started unironically believing.